1. Field of the Invention
The present invention relates, in general, to mobile communication technology and, more particularly, to a communication system composed of different kinds of base stations such as a macro base station and a femto base station and a method for controlling unfavorable interference, occurring due to the different kinds of base stations.
2. Description of the Related Art
Normally a femtocell refers to a small cell that covers a very small range in a cellular system. A small cellular base station for controlling such femtocells is referred to as a femto base station or Home Node B (HNB). Such a femto base station is designed as a base station for use in residential or small business environments. Since a femtocell generally operates in an environment where a macro cell is also operating, unfavorable interference may often occur depending on the location of user equipment such as a mobile communication device, e.g., a cellular telephone. An example is shown in FIG. 1.
FIG. 1 is a schematic view illustrating a conventional communication environment in which interference occurs between a macro base station and a femto base station.
Referring to FIG. 1, the first femtocell A controlled by the first femto base station 100a (also referred to as HNB A) and the second femtocell B controlled by the second femto base station 100b (also referred to as HNB B) are located between the first macrocell A controlled by the first macro base station 200a and the second macrocell B controlled by the second macro base station 200b. 
Additionally, certain user equipment 300a (also shown as UE macro) is located in the first macro cell. In the first femtocell A, certain user equipment 300b (also shown as UE A1) is located near the first macro cell, and another user equipment 300c (also shown as UE A2) is located near the first femto base station 100a. Also, user equipment 300d (also shown as UE B1) is located in the second femtocell, and user equipment 300f (also shown as UE macro) is located near the second femtocell in the second macro cell.
In FIG. 1, dotted lines indicate a downlink of data transmission from a base station to user equipment, and solid lines indicate an uplink of data transmission from user equipment to a base station.
A certain uplink between the first macro base station 200a and user equipment 300a may be affected by interference with another uplink, indicated by number 1, between the first macro base station 200a and user equipment 300b. Furthermore, a certain downlink between the second macro base station 200b and user equipment 300f may be affected by interference with another downlink, indicated by number 2, between the second femto base station 100b and user equipment 300f. 
In order to control such interference between a macro base station and a femto base station, various methods have been studied in the art. For instance, widely used methods of controlling such interference are a power control method which regulates the transmission power (also referred to as Tx power) of a femto base station, an adaptive Tx on/off method which turns on the transmission power of a femto base station when the user equipment approaches a femtocell, and a radio resource management (RRM) method in an Orthogonal Frequency Division Multiplexing (OFDM) system.
Among them, an adaptive Tx on/off method is implemented as follows. A femto base station is composed of a transmission terminal for transmitting data and a reception terminal for receiving data. A femto base station monitors the reception signal strength from user equipment while turning off the power of a transmission terminal and turning on the power of a reception terminal. When a user equipment approaches a femtocell, a femto base station checks the reception signal strength of the approaching user equipment at a reception terminal. If the signal strength is more than a threshold value, the femto base station turns on the power of the transmission terminal in order to reduce interference.
Meanwhile, an RRM-based method controls interference by appropriately allocating resources to a femto base station and a macro base station. In this case, resources of the femto base station and the macro base station are allocated through a scheduler at the macro base station.
Unfortunately, such conventional methods for controlling interference have some shortcomings. For instance, since interference in user equipment due to a femto base station may occur irregularly, it is difficult to react to interference at a suitable time. Additionally, because it may be unclear which femto base station influences an user equipment by interference, it is difficult to properly allocate resources to a femto base station and a macro base station.